All Volcano events are active in 100% of games. They can only occur on visible tiles.
New Volcano: (iWeight: 20):
Any tile that has no city and not bonus resource on it can erupt. Eruption adds an active Volcano to the plot and the plots around it gain 1

, unless they are coast or ocean. Improvements around the Volcano (and on it) are destroyed. Units on the Volcano plot are displaced by 1 plot and damaged to 10% of health, if there is no plot they can move into legally, they are killed. Note: this also can happen in coast and ocean tiles - which adds a land tile then.
Extinct Volcano erupts: (iWeight: 80):
Any tile with Obsidian or Sulphur has no city on it can erupt. Eruption adds an active Volcano to the plot, removes the bonus and the plots around it gain 1

, unless they are coast or ocean. Improvements around the Volcano (and on it) are destroyed. Units on the Volcano plot are displaced by 1 plot and damaged to 10% of health, if there is no plot they can move into legally, they are killed. Note: this also can happen in coast and ocean tiles - which adds a land tile then.
Dormant Volcano erupts: (iWeight: 300):
Any tile with a dormant Volcano (FEATURE_VOLCANO) can erupt. Eruption adds an active Volcano (FEATURE_VOLCANO2). Improvements around the Volcano (and on it) are destroyed. Units on the Volcano plot are displaced by 1 plot and damaged to 10% of health, if there is no plot they can move into legally, they are killed. Note: this also can happen in coast and ocean tiles - which adds a land tile then. If no previous eruption added 1

to the surrounding tiles, this is done now.
Active Volcano erupts: (iWeight: 1100):
Any tile with an active Volcano can erupt. Improvements around the Volcano (and on it) are destroyed. Units on the Volcano plot are displaced by 1 plot and damaged to 10% of health, if there is no plot they can move into legally, they are killed. Note: this also can happen in coast and ocean tiles - which adds a land tile then. If no previous eruption added 1

to the surrounding tiles, this is done now.
Active Volcano goes dormant: (iWeight: 400):
Active Volcano feature is switched to dormant Volcano feature.
Dormant Volcano goes extinct: (iWeight: 100):
Dormant Volcano feature is removed, the tile gains a rocky hill and Obsidian or Sulphur. The tiles around them revert to their normal yield, unless they are ocean or coast.
The original Volcano event is disabled. Note: this had an iWeight of 100 meaning that now there are 20 times as many chances for Volcano related events (combined iWeight of these events is currently 2000)...